Boosting photocatalytic water splitting of TiO2 using metal (Ru, Co, or Ni) co-catalysts for hydrogen generation

The photocatalytic activity of titanium dioxide (TiO2) nanoparticles toward hydrogen generation can be significantly improved via the loading of various metals e.g., Ru, Co, Ni as co-catalysts. The metal co-catalysts are loaded into TiO2 nanoparticles via different deposition methods; incipient wet impregnation (Imp), hydrothermal (HT), or photocatalytic deposition (PCD). Among all of the tested materials, 0.1 wt% Ru–TiO2 (Imp) provided the highest initial hydrogen catalytic rate of 23.9 mmol h−1 g−1, compared to 10.82 and 16.55 mmol h−1 g−1 for 0.3 wt% Ni–TiO2 (Imp) and 0.3 wt% Co–TiO2 (Imp), respectively. The loading procedures, co-catalyst metals type, and their loading play a significant role in elevating the photocatalytic activity of pristine TiO2 semiconductors toward hydrogen generation. Redox transition metals e.g., Co and Ni exhibit comparable photocatalytic performance to expensive elements such as Ru.


Incipient wet impregnation method (Imp)
TiO 2 was suspended in 3 mL using an alumina crucible.After that, the mixture was stirred by a glass rod and ultrasonicated.The suspension was powdered by placing the crucible over a hot water bath with constant stirring till complete dryness.The powder sample was then moved to a tube furnace and purged by Ar (99.99%) using a flow rate of 100 mL min −1 for 15 min.After the purge, H 2 (99.99%) gas flowed at a rate of 10 mL min −1 , and the furnace temperature was raised to 200 °C (at a rate of 10 °C min -1 ) and held for 1 h.The powder was cooled naturally under the flow of H 2 gas.

Hydrothermal method (HT)
A suspension of TiO 2 powder (1 g) was prepared in 90 mL of distilled water and then sonicated for 10 min.For loading the required ratio of metal to TiO 2 , the studied volume of the metal precursor was mixed with 5 mL ethanol and then added dropwise to the TiO 2 suspension, the reaction was maintained under vigorous stirring for one hour.The dispersion was heated at 180 °C for 12 h using a Teflon-lined stainless steel autoclave.The materials were collected and washed with water.

Photocatalytic deposition (PCD)
In the photo-deposition method, the reduction can be achieved via the photon energy-assisted methanol method.Briefly, the metal precursor was added to 200 mL of 20% methanol aqueous suspension containing TiO 2 and then transferred into a Pyrex cell.After the purge with Ar gas, the cell was subjected to a UV-LED lamp for three hours.The precipitate was collected, washed using distilled H 2 O, and dried.

Photocatalytic H 2 production experiments
The photocatalytic hydrogen generation was measured via an online system.As shown in Fig. S1, a Pyrex glass reactor connected to a flow system was set for photocatalytic evaluation of hydrogen gas.Typically, 50 mg of the photocatalyst was suspended in the reactor containing 200 mL of 20 vol.% methanol aqueous solution as a hole scavenger and subjected to constant stirring (800 rpm).Before irradiation, the suspension was ultrasonicated for 5 min and purged for 30 min with Ar gas.After that, the Ar gas flow rate was decreased to 10 mL min −1 , and the suspension cell was situated at a 1 cm distance from the illumination UV-LED light source (25 W, 365 nm, NVMUR020A, NICHIA, Japan).The amount of gas evaluated was detected every 15 min using a gaschromatography (GC) system (Shimadzu, GC-2014, Shin Carbon ST 80/100 with Length: 2 m and ID:2 mmID column, thermal conductivity detector, and argon gas as the carrier).The experiment lasted for 300 min at room temperature.
Photoelectrochemical measurements were performed using a potentiostat workstation (CorrTest ® Instruments, model CS350) an electrolyte of Na 2 SO 4 aqueous solution (40 mL, 0.1 M).The working electrode was prepared by casting the materials into FTO substrates with an active area of ca.1.0 cm 2 .The counter and reference electrodes were a Pt wire and Ag/AgCl, respectively.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were recorded with and without light.
The working electrodes were prepared using the electrophoretic deposition method.Typically, the photocatalyst (20 mg) was dispersed in 1 mL isopropanol and sonicated for 15 min to make a homogenous slurry or paste.50 µL of the slurry was deposited into an FTO conducting glass substrate that was then dried in air.The materials were deposited via layer-by-layer procedures.Finally, the electrodes were dried in an oven and calcinated at 120 °C for 60 min.

Characterization instruments
XRD patterns were collected using Bruker D8 advance equipment (Cu K α radiation).XPS spectra were recorded using Al K α radiation (Thermo Scientific, USA).TEM and HR-TEM images were collected using JSM-2100 (JEOL, Japan).The ultraviolet-visible (UV-Vis) DRS of powder samples were collected using an Evolution 220 spectrophotometer (Thermo Fisher Scientific, UK).Tauc's plots were used to determine the materials band gap.Nitrogen adsorption-desorption isotherms were determined using Quantachrome (USA, at 77 K).Pore size distribution (PSD) was determined using the Barrett-Joyner-Halenda (BJH) method.
TEM and HR-TEM images for Ru/TiO 2 were recorded for loading methods of Imp, HT, and PCD (Fig. 3).Two different particles are observed in TEM images corresponding to TiO 2 and Ru (Fig. 3).Irregular particles of TiO 2 with a particle size of 10-50 nm (Fig. 3).The particles of TiO 2 can be confirmed from the lattice fringes observed using HR-TEM images (Fig. 3).TiO 2 displays lattice fringes of 0.33 nm (101).The high electron density in Ru causes the observation of dark particles located in the plane of (101) for TiO 2 (Fig. 3).N 2 adsorption-desorption isotherms and PSD of the materials were recorded as shown in Figs.S8 and S9, respectively.Data analysis shows

TiO 2
Purge with (Ar) for 15 ) BET-specific surface areas of 70-203 m 2 /g with a pore size of 1.5-2 nm (Figs.S8, S9).It is important to mention that most of these porosities refer to the interparticle pores formed between metal-loaded and TiO 2 crystals i.e., interparticle porosity according to TEM images (Fig. 4).
The optical absorption of the materials was determined using DRS (Fig. 4, Figs.S10, S11).The band gap values were determined using Tauc's plots.UV-Vis spectra of all materials exhibit the characteristic absorption peak for TiO 2 at maximum absorption at wavelength 300 nm (Fig. 4, Figs.S10, S11).TiO 2 shows a bandgap of 2.97 eV (Fig. 4b).Transition elements cause a decrease in the bandgap values to a lower value of 2.6-2.8eV.The drop in the bandgap of TiO 2 indicates the formation of heterojunction between the transition elements and TiO 2 nanoparticles.This effect can tune the material's photocatalytic performance.

Photocatalytic water splitting
The photocatalytic activity of TiO 2 -based composite is tested.The effects of metal types (e.g.Ru, Co, and Ni), loading procedures (Fig. 5), and loading percentage (Fig. S12) were investigated.The HGR can be arranged in the sequence of Ru > Co > Ni (Fig. 5).HGR values for Ru/TiO 2 , Co/TiO 2 , and Ni/TiO 2 were 23.91, 16.55, and 10.83 mmol/g h, respectively.Electron-rich elements such as Ru exhibit high HGR.
The initial and cumulative hydrogen rate was observed for 0.1, 0.3, and 0.3 wt.% of Ru, Ni, and Co metals, respectively.High loading of these metals causes a decrease in the initial and cumulative rates.These observations could be due to the light block caused by high loading that prevents light radiation from reaching the external surface of TiO 2 semiconductors.
The effect of the loading method was investigated using Imp, HT, and PCD (Fig. 5, Fig. S12).The methods of metal loading affect the distribution of the co-catalyst e.g.Ru, Ni, and Co.Thus, they affect the composite's catalytic performance.Among the three loading methods, Imp exhibits a high catalytic performance (Fig. 5, Fig. S12).The high photocatalytic performance of the materials synthesized using Imp could be due to the homogenous distribution of the loaded metals into TiO 2 according to TEM images (Fig. 3).The decrease of the bandgap of TiO 2 using the Imp procedure is another explanation of this observation (Fig. 4).Further investigations were recorded using electrophotochemical measurements via EIS (Fig. 6) and CV curves (Fig. 7 and Fig. S13).
EIS spectra using the Nquists plot of all materials were recorded with and without light radiation (Fig. 6).Based on the analysis of Nquists plots, the small circle indicates low impedance i.e., high conductivity.All metal types i.e., Ru, Co, and Ni exhibit small circles for Imp compared to other loading procedures The same observation can be noticed without (Fig. 6a-c) and with light radiation (Fig. 6d-f).Furthermore, the size of all circles is smaller under UV radiation compared to without light.Based on EIS analysis using Nyquist plots (Fig. 7), the co-catalysts of Ru, Ni, and Co exhibit small circles compared to bare TiO 2 .The presence of these cocatalysts increases the conductivity of the semiconductor TiO 2 .This observation were noticed without light radiation (Fig. 7a) and with light radiation (Fig. 7b).There is a dramatic decrease in the circle size indicating high conductivity for the composites compared to the bare TiO 2 .CV curve for bare TiO 2 and Ru_TiO 2 exhibit only cathodic reduction profile (Fig. 7c).On the other side, Co_TiO 2 and Ni_TiO 2 exhibit redox properties i.e. reduction-oxidation peaks.All prepared photocatalysts by the Imp method showed higher cathodic current than the others prepared by HT and PCD methods (Fig. S13).
The mechanism of H 2 generation via water splitting requires a photocatalyst with a negative conduction band potential higher than the values for water reduction potential (− 0.41 eV).The photocatalyst should also display a positive valence band value than the water oxidation potential i.e., 1.23 eV.TiO 2 semiconductor displays suitable band gap for water splitting.However, the high rate for the recommendation of the created electron-hole.Thus, bare TiO 2 exhibits lower HGRs (Fig. 5).Moreover, we can improve the photocatalytic performance via the addition of co-catalysts such as Ru, Co, and Ni (Fig. 5).These co-catalysts improved also the conductivity (Figs. 6, 7) of pristine TiO 2 enabling high electrochemical performance that can be reflected in high HGR values (Fig. 5).
A summary of some photocatalysts used for water splitting is tabulated in Table 1.Our study reported several parameters that can enhance the photocatalytic performance of well-known semiconductors i.e., TiO 2 .The co-catalysts properties such as metal types, loading, and deposition procedures play significant roles in the materials' performance.Our photocatalyst composites exhibit high HGRs compared to several reported materials (Table 1).Aqueous solutions of the chloride salts of Ru, Pd, and Ag were impregnated into TiO 2 anatase with a content of 30 wt.% (Table 1) 37 .RuO 2 \TiO 2 exhibited higher photocatalytic activity compared to PdO\TiO 2 and Ag 2 O\TiO 2 37 .Ni\Pt@TiO 2 (110) offered high photocatalytic HGRs from methanol alcohols under ultrahigh vacuum (UHV) 38 .Ni (2.21 wt.%)/Pt/black TiO 2−x was synthesized via photodeposition (Table 1) 39 .The composite contains metallic Ni and Pt (i.e., 2 at.%) 39 .The material exhibited high charge separation efficiency.It showed HGRs of 69 and 3.1 mmol g -1 h -1 under UV-Vis and visible light, respectively (Table 1).The presence of Pt was essential to obtain metallic Ni instead of Ni(OH) 2 (Table 1) 39 .The prepared photocatalysts in our study using inexpensive metals e.g., Co and Ni exhibit comparable HGRs to expensive co-catalysts such as Ru (Table 1).The www.nature.com/scientificreports/high performance of these transition elements is mainly due to their high electrochemical performance.Cocatalysts such as Ni or Co nanoparticles enhanced the charge migration and separation of photogenerated species on TiO 2 40 .These metal species i.e., Ni 2+ improved the electron transfer inside TiO 2 due to their low potential (Ni 2+ + 2e − = Ni, E o = − 0.23 V) 41 .Thus, different forms of Ni were reported including Ni 42,43 , Cu-Ni 44 , Ni-Pd 45 , NiO [46][47][48][49] , and hydroxides (Ni(OH) 2 , Table 1) 50 .The presence of oxygen vacancy inside the composite improved the photocatalytic performance of TiO 2 .A study showed that CoO/h-TiO 2 exhibited Z-scheme heterostructures with oxygen vacancy 51 .The authors observed that these vacancies were created during the composite formation and enhanced the photocatalytic H 2 generation of TiO 2 with an HGR value of 129.75 μmol h -1 (Table 1) 51 .However, the synthesis procedure required several steps and calcination at high temperatures.On the other side, our synthesis protocols are simple.Our catalysts exhibit high HGR values (Table 1).

Conclusions
This work presents an effective approach was reported to improve the photocatalytic performance of TiO 2 nanoparticles for the production of hydrogen.The use of several metal co-catalysts e.g., Ru, Co, Ni applied in diverse ways (incipient wet impregnation, hydrothermal, and photo-deposition) resulted in a substantial enhancement in hydrogen generation compared to the pristine TiO 2 .Ru-TiO 2 , synthesized using the incipient wet impregnation method, exhibited the highest initial rate of hydrogen evolution (i.e., 23.9 mmol h −1 g −1 ), exceeding both

Figure 6 .Figure 7 .
Figure 6.(a-c) Nyquist plots of the EIS data in dark conditions for Ru_TiO 2 , Co_TiO 2 , Ni_TiO 2 nanocatalysts of different metal loading methods and (d-f) represent Nyquist plots of the EIS data under UV light illumination in 0.1 M Na 2 SO 4 electrolyte.

Table 1 .
Summary of TiO 2 -based photocatalysts used for water splitting.